1. Field of the Invention
This invention relates generally to flowmeters and, more particularly, to a method and apparatus for validating the accuracy of a flowmeter in situ.
2. Discussion of the Related Art
There are many types of flowmeters which are employed to measure a variety of flowing fluids through conduits of a myriad of sizes and shapes. Depending on the particular situation, accuracy of the outputs of a flowmeter may range from useful, to important, to extremely critical. The accuracy of such a meter is ultimately dependent upon the proper functioning of the sensors or other signal-producing elements which have an active relationship with the flowing fluid.
In order to be confident that a flowmeter is functioning properly and providing accurate information, the sensors must be tested for proper operation on a periodic basis. Where accuracy is extremely critical, validation of sensor functioning will typically be more frequent than where the flowmeter provides useful, but not normally critical, flow information.
Routine verification of flowmeter calibration and the traceability of information are key to current auditing and regulatory requirements. In the past, this has proved difficult, time consuming, and costly. For example, in the water industry, the task could entail mechanical excavation and removal of the flowmeter resulting in disruptions of the water supply to the local community.
There are generally two known methods to verify the calibration of flowmeters. One is to remove the flowmeter from the process or installation and send it to a qualified laboratory for verification. The other method is to install or connect by means of a bypass, on a temporary basis, a known reference flow measurement standard in series with the meter being tested. This is an in-situ verification.
Both of the above methods are referred to as “wet,” or complete sensor-to-output, checks of the meter being tested.
In some installations it may only be necessary to check a selected portion of the sensor-to-output signal path. For example, the transmitter electronics of a thermal dispersion flowmeter could be checked by employing precision decade resistance boxes as a substitute for the thermal flow element, and adjusting the decade resistance boxes to verify the differential-resistance-input-to-current or other output relationship of the sensor or flow element portion of the transmitter. This method is relatively incomplete and is generally less desirable because it only synthesizes the flow element input.
Similarly, an artificial differential pressure could be introduced to, for example: a venturi meter, a pitot tube array, or an orifice based meter flow element to determine that the output corresponds correctly to the synthesized input signal pressure difference.
This partial approach is known as a “dry” calibration. Although dry calibration checks are typically less accurate and less complete than wet calibration checks, in many instances dry checks are tolerated because they are more convenient and less expensive than wet checks. Their great disadvantage is they do not check the validity of the flow element input signal and rely, as illustrated above, on a synthetic input.
Wet calibration procedures as described above are expensive, inconvenient, time consuming, and require skilled operators in order to produce good results. In general, dry checks, as have been previously known, are less accurate and may not be an available option for every flowmeter. Their greatest drawback is they fail to check the most vulnerable element in the system, the primary element in the flow stream, which is the sensor or flow element.